OSTRACODE DELTA-O-18 AND DELTA-C-13 EVIDENCE OF HOLOCENE ENVIRONMENTAL-CHANGES IN THE SEDIMENTS OF 2 MINNESOTA LAKES

Stable oxygen and carbon isotope geochemistry of ostracode valves, abundance and assemblages of ostracode species, and sedimentological parameters from cores taken in Williams and Shingobee Lakes in north-central Minnesota show changes in climatic and hydrologic history during the Holocene. Isotopic records are consistent with the following scenario: Before 9800 yr B.P. the two lakes were connected. Increasing evaporation through the jack/red pine period (9800-7700 yr B.P.) led to lower lake levels, leaving small separated basins. The prairie period (7700-4000 yr B.P.) reflects high aridity, and lake levels reached low stands shortly before 6500 yr B.P. Low lake levels are associated with groundwater discharge between 6500 and 6000 yr B.P. The hardwood period (4000-3200 yr B.P.) corresponds to long cold winters and warm to cool summers with lower evaporation rates and slower sedimentation. During the white pine period (<3200 yr B.P.) evaporation increased and/or precipitation shifted to the summer months. These changes can be related to shifting atmospheric circulation patterns. Zonal flow was probably dominant during the early Holocene until the end of the prairie period (c. 4000 yr B.P.). During the hardwood period a combination of zonal and meridional flow patterns caused long and cold winters and wetter summers. During the white pine period winters were shorter and the meridional flow pattern more significant. Today meridional flow dominates the circulation pattern.